Solar cells have received attention as inexhaustible, renewable, and environmentally friendly electrical energy sources. At present, first generation crystalline solar cells using inorganic materials (typified by silicon crystalline solar cells) account for 90% of the photovoltaic power generation market. However, first generation crystalline solar cells generate electricity at 5 to 20 times higher cost than coal, petroleum, and natural gas and are thus not considered profitable. Under these circumstances, second generation crystalline solar cell technologies have emerged as promising alternatives to first generation crystalline solar cells. The total market share of second generation thin-film solar cells, including silicon (5.2%), CdTe (4.7%), and CIGS (0.5%) thin-film solar cells, is estimated to be 10% but is still unsatisfactory. Second generation solar cell technologies involve troublesome device fabrication processes and require expensive equipment, incurring considerable costs. The increased costs are mainly attributed to processes for providing semiconductor thin films under vacuum at high temperature. Thus, organic polymer solar cells are investigated as new possibilities because they can be fabricated at greatly reduced costs by low temperature solution processing. Organic polymer-based energy materials possess low power conversion efficiency values compared to inorganic materials but are gaining more importance due to their many advantages, such as ease of device fabrication, mechanical flexibility, ease of molecular design, and low price.
The biggest challenge in organic solar cells using organic semiconductors, such as conjugated polymers, is lower power conversion efficiency than conventional solar cells using inorganic semiconductors. For this reason, organic solar cells using organic semiconductors have not yet been put into practical use. There are three major reasons for the low power conversion efficiency of conventional solar cells using conjugated polymers. The first reason is low sunlight absorption efficiency. The second reason is that excitons generated in organic semiconductors by sunlight are difficult to separate into electrons and holes due to their high binding energy. The third reason is that traps capable of readily capturing carriers (electrons and holes) are easy to form, causing low carrier mobility. Semiconductor materials are generally required to have high carrier mobility. Nevertheless, conjugated polymers suffer from the problem of lower mobility of charged carriers than conventional inorganic crystalline semiconductors and amorphous silicon.
For these reasons, developments of approaches to absorb much sunlight and facilitate the separation of created electrons and holes from excitons and approaches to improve the mobility of carriers by suppressing scattering of carriers or capture of carriers by traps between amorphous areas and chains of conjugated polymers are crucial keys for practical application of organic semiconductor solar cells.
Photovoltaic devices using organic semiconductors known hitherto are generally classified into the following device constructions, for example: Schottky junctions where electron donating organic materials (p-type organic semiconductors) are bonded to low work function metals; and heterojunctions where electron accepting organic materials (n-type organic semiconductors) are bonded to electron donating organic materials (p-type organic semiconductors). These devices have low power conversion efficiency values because only the organic layers (water molecule layers) around the junctions contribute to photocurrent generation. Further improvements are thus needed in power conversion efficiency.
One approach to power conversion efficiency improvement is to develop bulk heterojunction photovoltaic devices in which electron accepting organic materials are mixed with electron donating organic materials and junction areas are increased to contribute to power conversion. Among them, photovoltaic devices were reported in which conjugated polymers are used as electron donating organic materials and semiconductor polymers having n-type semiconductor characteristics, for example, C60 fullerene derivatives, are used as electron accepting organic materials.
Organic solar cells based on bulk heterojunction structures have recorded low efficiencies of at most ˜1% in the early stage of development but have achieved efficiencies of 4-6% by the use of P3HT as a photoactive layer material. Since low band gap organic materials were invented, organic solar cells with high efficiencies of ≧7% have been reported. Low band gap polymers undergo intramolecular charge transfer (ICT) from electron-rich monomers to electron-deficient monomers. This phenomenon leads to a reduction in band gap, resulting in efficient absorption of sunlight over a broad range of wavelengths.
The most serious problem of current polymer solar cells is considerably low power conversion efficiency compared to that (about 20%) of structures using inorganic materials, such as silicon and CIGS. Active layers serving to convert absorbed sunlight to electricity play an important role in the development of highly efficient organic solar cells. There is thus a need to develop novel active layer materials with higher efficiency through processing condition optimization. To this end, it would be essential to develop low band gap polymers with broad absorption bands and high molar extinction coefficients capable of effectively absorbing sunlight, improve hole and electron mobilities for excellent photocurrent characteristics, control the electronic structure of polymers for high open-circuit voltage, and develop multilayer polymer solar cell technologies.